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from the straight line, or tangent, along which, if left alone, itwould at any moment proceed, into the curve in which itactually moves. The power of the Sun to produce this effectenables it to pull the Moon away from such a straight linetowards itself in each successive second of time, through adistance equal to about myths (or nearly ^hths) of an inch.In one minute the effect would not be 60 times as great, but,according to the law which holds in such cases, 60 times 60, or3,600 times ; so that the pull of the Sun upon the Moon wouldin one minute deflect it from a rectilineal path through adistance of about 35 feet. The Sun is of course constantlyexerting a similar power upon the Earth , and curving its orbitalmost exactly to the same extent. But as far as the Earth 'sattraction upon the Moon is concerned, apart from that of theSun , the orbit of the Moon round the Earth is only curved somuch as to draw it towards the Earth out of motion in astraight line to a distance equal to about T 4ir<yths (or about■jVths) of an inch per second, or about 16^ feet in a minute.
Remembering this, let us first of all consider the circum-stances of the Moon ’s motion when it is in the position ofNew Moon . Let the Moon be situated at m (Fig . XI.), which
Fig . XI.— Showing how the attractions of the Sun and of the Earth respectivelycurve the orbit of the Moon .
we may, for simplicity, imagine to be exactly in the straightline, es, joining the Earth (which is supposed to be in thedirection me) and the Sun (which is supposed to be in thedirection ms). The Moon has, as we have explained, a velocityof about 66,000 miles per hour from west to east round theSun . In one minute it would consequently move from m to f,in a direction perpendicular to ms, through a distance of about1,100 miles, if no attraction of the Sun or Earth continued to